In New York and Pittsburgh, incumbents are expected to win. Atlanta is poised to elect its first white mayor in more than 30 years. And Houston could elect its first openly gay or first Latino mayor.

ATLANTA AND LOS ANGELES — By a surprisingly narrow margin, voters chose to reelect billionaire Michael R. Bloomberg for a third term as New York mayor Tuesday, one of a number of big-city electoral decisions made amid predictions of continuing financial stress for municipal governments nationwide.

With all precincts reporting, Bloomberg, a Republican turned independent, had earned 51% of the vote in his race against Democrat William Thompson Jr., the city's comptroller. Thompson earned 46% of the vote.

It was closer than expected, especially given the discrepancy in spending that defined the race. Bloomberg -- a former media mogul listed by Forbes as the eighth-richest American -- spent more than $100 million of his personal fortune on his campaign, outspending the challenger 10 to 1.

Other close mayoral contests were unfolding Tuesday in Atlanta and Houston. In both cities, it appeared possible that voters had whittled large fields of contenders to two-candidate races that could be decided in runoffs.

Bloomberg, 67, began laying the groundwork for Tuesday's victory last October, when he successfully campaigned to have a 15-year-old term-limit law lifted.

He told New Yorkers he would use his financial savvy to help guide the city through the financial meltdown -- a message that resonated with Brandon Arbiter, 25, who lives in Gramercy Park.

"Having a mayor with such experience with the finance industry is beneficial right now," Arbiter said.

"He has shown that someone with tremendous business acumen can come into a government agency and make it run more effectively."

Earlier this year, Bloomberg had already proposed cutting the city budget by $1 billion and raising taxes to help address a $4-billion budget shortfall.

Like many other cities, Atlanta is suffering from budget problems, which have been compounded by numerous accounting blunders by city bureaucrats.

A focus on those problems helped Mary Norwood, an eight-year city councilwoman, establish herself as a front-runner in the majority-black city, giving her the chance to become the city's first white mayor since 1973.

The Atlanta race was too close to call late Tuesday, but with 60% of precincts reporting, Norwood was ahead with 44% of the vote, leading African American contenders Kasim Reed, with 38%, and Lisa Borders, with 14%.

Reed, a former state senator, received major endorsements from the city's civil rights establishment, and attacked Norwood in the last days of the campaign as a closet Republican -- a perceived liability in this Democratic-leaning city.

If no candidate garners more than 50% of the vote, the top two will meet in a Dec. 1 runoff.

In Houston, City Comptroller Annise Parker and former city attorney Gene Locke were headed for a runoff. Parker led a field of four with nearly 31% of the vote, and Locke had 25%. Parker's victory would be a different milestone: She would be the city's first openly gay mayor.

The runoff date has not been set, but it will be in December.

Even before the votes were counted, Parker's supporters heralded her campaign as a victory for the mainstreaming of gay rights in Houston, a relatively liberal redoubt in a famously conservative state.

Observers had also predicted that Parker's financial experience as comptroller would serve her well in the current economic climate.

"Annise is a known entity and a liked one," said Ray Hill, a longtime gay activist in Houston.

"She's fiscally cautious and socially liberal, and that matches the general majority tenor of inner-city Houston."

In Pittsburgh, three candidates under the age of 34 vied for the top office.

Ravenstahl, the incumbent, had been expected to win handily after easily defeating an opponent in the Democratic primary this spring, said George Krause, a professor of political science at the University of Pittsburgh.

Ravenstahl beat out two independents: lawyer Kevin Acklin, 33, and Franco Harris, 30, a businessman whose father is the former Pittsburgh Steelers star running back of the same name.

Ravenstahl became the youngest mayor in Pittsburgh's history at 26, when the previous mayor died and, as City Council president, he inherited the seat.

He was elected on his own right in a 2007 special election, winning 62% of the vote.

During the campaign, Ravenstahl boasted that he had moved Pittsburgh from "the brink of bankruptcy to a city that is being spotlighted for its fiscal sobriety."